Receiving Money...what does "pending" mean?

Receiving Money...what does "pending" mean?

I'm trying to sell a laptop to a guy in the UK. I told him I would send it after I got the money in my paypal and he agreed. He said he already got the e-mail from paypal and that he already sent me the money. I got an e-mail from paypal that said "We'll send you a confirmation email as soon as you get paid." but I haven't gotten another e-mail from paypal since then. In my paypal it says "Pending" under Payment Status. What's going on? Can someone explain this to me.

P.S.- I have never used paypal before and I have no idea about how this works or what's going on with the Pending status.